The fabrication of printed circuit boards includes producing a prepreg of a fiber sheet such as woven cloth or non-woven fabrics for instance, of glass fiber as a reinforcing material impregnated with a thermosetting synthetic polymer such as an epoxy resin composition or polyimide resin composition. A plurality of such prepregs are laminated and a metal foil is placed on each of the major surfaces of the laminate, and the assembly is then hot pressed.
A circuit wiring is formed on the metal foil on each of the major surfaces of the substrate by photolithographic technique. The photolithographic process involves providing a photoresist that is exposed to UV light for processing. In fabricating the circuits by additive pattern plating, UV light should not be transmitted through the laminate from one side of the printed circuit board to the other side. In particular, stray light transmittance can expose photoresist on the other side of the printed circuit board and can cause defects in the circuitry. Accordingly, various dyes and colored resin additives have been added to the laminate polymer in order to absorb UV light so that it does not pass through the laminate. However, as printed circuit board component cores become thinner and thinner, it has been observed that a light transmittance path has surfaced that formerly was not a problem. In particular, open and near open circuit defects have been observed which are believed to be caused by stray light. Such defects have been located over intersections of the glass cloth weave referred to as knuckles. Knuckles are thicker than surrounding areas of glass cloth and can stack up on top of each other somewhat randomly. Such stacking produces resin poor, vertically oriented zones through which UV light can be transmitted. Furthermore, it has been observed according to the present invention that the glass fibers may act in the nature of a light pipe to collect UV light from non-knuckle areas and transmit it to the surface by a fiber optic-like effect.
Moreover, during additive circuitization processing, bleedthrough has been observed on the first side of the laminate due to UV light from exposure of the second side. It has been believed that the depletion of photoinhibitors and/or oxygen in the first side photoresist from the first side exposure, effectively increases the photospeed of the remaining unexposed first side photoresist. When this sensitized resist sees even a small dose of UV transmitted through the glass knuckles from the second side exposure, it polymerizes, does not develop off and causes defects. As cores become thinner and thinner, the bleedthrough problem adversely affects the photoresist on both sides of the laminate. It would therefore be desirable to provide a process for overcoming these problems.